The present invention relates to the fabrication of integrated circuits. More particularly, the invention provides an electrostatic chuck having an improved dielectric coating and a method for fabricating the improved dielectric coating.
Electrostatic chucks are devices which have gained wide usage in the semiconductor industry for clamping semiconductor wafers during manufacturing processes, such as high-density plasma reactions. Electrostatic chucks employ an electrostatic force between oppositely charged surfaces to secure the wafer to the chuck. Fabrication of electrostatic chucks typically involves machining a process-compatible metal, such as aluminum or anodized aluminum, into a suitable support pedestal and grit-blasting the top surface of the pedestal. A layer of dielectric material, such as ceramic, is then typically applied to the upper surface of the pedestal and ground to a smooth, planar upper surface for supporting the wafer. During processing, a voltage is applied between the wafer and the metal pedestal, generating an opposing charge on either side of the dielectric layer. This opposing charge creates an attractive, substantially uniform, coulomb force therebetween that secures the wafer to the dielectric layer.
One important consideration in the manufacture of electrostatic chucks is the electrical properties of the layer of dielectric material, such as its dielectric strength and dielectric constant. For example, the clamping force of an electrostatic chuck, i.e., the force with which the wafer is held in place on the chuck, generally increases with the square of the applied voltage to the chuck (for a given dielectric layer thickness). In typical processes, a relatively large voltage is applied across the electrostatic chuck to clamp the semiconductor wafer to the chuck during processing. This large clamping voltage is usually required because cooling gas is delivered between the semiconductor wafer and the chuck to decrease the temperature of the wafer. This cooling gas impinges on the back side of the wafer and lifts the wafer away from the chuck. Increasing the dielectric strength of the dielectric layer increases the capability of the dielectric layer to withstand higher applied voltages without causing dielectric breakdown across the layer and subsequent failure of the electrostatic chuck.
Another important consideration in the manufacturing of electrostatic chucks is the porosity of the dielectric layer. A porous dielectric layer is less resistant to bombardment from plasma during processing and, therefore, may wear down relatively quickly, increasing the downtime and decreasing the throughput of the wafer manufacturing process. Porosity also has an adverse effect on the electrical characteristics of the dielectric layer, i.e., the layer's electrical stability and dielectric strength. In addition, a porous dielectric layer tends to absorb moisture and/or gases from the chamber into the open pores of the dielectric layer. Moisture and gas build-up within these pores further degrades the electrical characteristics of the dielectric layer.
Dielectric layers are typically formed by plasma spraying a ceramic powder, such as aluminum oxide, onto the upper surface of a metallic pedestal. In the plasma spray process, an electric arc is established between a pair of spaced electrodes, and gas is directed in contact with one of the electrodes so that the gas contains an electric arc. The powder used to produce the coating is mixed with the arc-containing gas and the mixture is constricted through a nozzle and deposited on the pedestal to form the dielectric layer. Although the plasma spraying process produces adequate dielectric layers, it would be desirable to improve the electrical properties of these dielectric layers to thereby improve the performance and increase the lifetime of electrostatic chucks.
In addition, it would be desirable to improve the yield produced during the manufacturing of electrostatic chucks (i.e., the ratio of the amount of serviceable electrostatic chucks to the total amount of chucks produced). One factor that reduces the yield of chucks produced by the plasma spray process is metallic contamination during processing. Metal particles are occasionally removed from the electrodes during plasma spraying and mixed into the powder that forms the dielectric layer. Metal particles on the surface of the dielectric layer may damage the semiconductor wafer during processing. In addition, the metal particles may provide a conductive path that allows voltage to arc through the dielectric layer. For these reasons, electrostatic chucks having a critical amount of metal contaminants in the dielectric layer are typically considered unserviceable, which reduces the overall yield and increases the cost of manufacturing the electrostatic chucks.